‘Bell curve’ normally refers to a Gaussian distribution. That’s so common that it’s also called the normal distribution. It’s very common because it emerges any time you’re looking at the sum of many things from a single distribution: I.e. lots of tiny fluctuations which, under the Central Limit Theorem, add up to a Gaussian distribution. 

None of the examples here are actually Gaussian, however. 

Of the three, the Maxwell distribution comes closest. It’s a little higher in the tails than a Gaussian. Physically, all the collisions aren’t quite equal: faster particles collide a bit more often. 

The other two distributions are even further than Gaussian. 

The Beta decay shape come from phase space: when the beta particle has a middling energy, there are lots of possibilities for the direction and energy of the nucleus and neutrino. At very high or very low energies, however, there are much fewer possibilities: everything has to line up just right, so the probability is lower. 

Still, many physical distributions do have a “round central hump, declining on both sides” look. When you’re combining a bunch of little effects, it’s unlikely they’ll _all_ go one way or the other:  having all the events push you out into one tail or the other is unlikely, and the further out you go, the less likely that lineup gets. So it’s common for a physical distribution to slope away from a central peak that’s roughly where all the +/- fluctuations have cancelled out.